Novel polyesters prepared from mixture of hydroquinone and bisphenols

ABSTRACT

A linear copolyester having a reduced specific viscosity of at least 0.5 is prepared from terephthalic or isophthalic acid, a bisphenol and hydroquinone. The ester unit derived from terephthalic or isophthalic acid and hydroquinone is contained in the copolyester in a proportion of 5 to 35 mole%. The copolyester has superior crack resistance, thermal stability, transparency and chemical resistance.

This application is a continuation-in-part application of copending U.S.Ser. No. 619,495 filed on Oct. 3, 1975.

This invention relates to improved polyesters having superior crackresistance, thermal stability, transparency and chemical resistance.

Polyethylene terephthalate has gained widespread commercial acceptancebecause of its superior mechanical properties and chemical resistance,but possesses unsatisfactory dimensional stability and transparencybecause its heat distortion temperature is relatively low and its rateof crystallization is fast.

Polycarbonate resins, on the other hand, are known as materials having ahigh heat distortion temperature and superior transparency, but have adefect of unsatisfactory chemical resistance.

In an attempt to remedy such defects of the conventional polyethyleneterephthalate, a method has already been proposed to polycondenseterephthalic acid, isophthalic acid and/or the derivatives of these withdihydroxy aromatic compounds such as 2,2-bis(4-hydroxyphenyl) propane.It is known that the resulting aromatic polyesters have a high heatdistortion temperature and superior transparency, and possess betterchemical resistance than the polycarbonate resins. Our detailedinvestigations, however, have led to the discovery that although thechemical resistance of such aromatic polyesters is better than that ofthe polycarbonate resins, these aromatic polyesters immediately whitenin highly polar solvents such as acetone to lose their transparency, andalso are susceptible to cracks upon application of flexural stress inthe presence of solvent.

Accordingly, an object of this invention is to provide improvedpolyesters having superior crack resistance, thermal stability,transparency and chemical resistance.

Another object of this invention is to provide a process foradvantageously producing such polyesters.

With a view to achieving the above objects, we have made extensiveinvestigations, and found that by copolymerizing a specified proportionof hydroquinone with the aromatic polyesters described above, there canbe obtained polyesters which have not only superior thermal stabilityand transparency but also resistance to solvents such as acetone, andcrack resistance.

The invention provides a linear copolyester having a reduced specificviscosity of at least 0.5 and consisting essentially of (a) an esterunit derived from at least one aromatic dicarboxylic acid selected fromterephthalic acid and isophthalic acid and hydroquinone and (b) an esterunit derived from at least one aromatic dicarboxylic acid selected fromterephthalic acid and isophthalic acid and a bisphenol of the generalformula ##STR1## wherein R₁ and R₂, independently from each other,represent an alkyl group containing 1 to 6 carbon atoms, and may belinked to each other, the proportion of said ester unit (a) being 5 to35 mole%.

Examples of preferred bisphenols of the general formula (I) aboveinclude 2,2-bis(4-hydroxyphenyl)propane (called bisphenol A),1,1-bis(4-hydroxyphenyl)cyclohexane (called bisphenol Z),bis(4-hydroxyphenyl)methane, and 2,2-bis(4-hydroxyphenyl)butane. Ofthese, bisphenol A is most suitable.

The copolyesters in accordance with this invention are mostcharacteristic in that a limited amount (5 to 35 mole%) of hydroquinoneis copolymerized with a polyester derived from terephthalic and/orisophthalic acid and the bisphenol of formula (I), whereby the chemicalresistance and crack resistance of the polyester are improved.

Up to 20 mole% of the ester units derived from terephthalic and/orisophthalic acid may be replaced by a unit derived from anotherdicarboxylic acid, for example, aromatic dicarboxylic acids such asnaphthalenedicarboxylic acid, diphenyldicarboxylic acid,methylterephthalic acid, diphenoxyethanedicarboxylic acid,methylisophthalic acid, diphenyletherdicarboxylic acid ordiphenylsulfonecarboxylic acid, aliphatic dicarboxylic acids such assuccinic acid, adipic acid or sebacic acid, cycloaliphatic dicarboxylicacids such as hexahydroterephthalic acid or hexahydroisophthalic acid;or an ester unit derived from a hydroxycarboxylic acid such asp-hydroxybenzoic acid or hydroxycaproic acid.

Furthermore, up to 20 mol% of the ester unit derived from the bisphenolof formula (I) may be replaced by an ester unit derived from anotherdiol, for example, aliphatic diols such as ethylene glycol, propyleneglycol, trimethylene glycol, tetramethylene glycol, neopentyl glycol orhexaethylene glycol, alicyclic-containing aliphatic diols such ascyclohexane dimethylol or tricyclodecane dimethylol, and aliphatic diolscontaining an aromatic ring in the molecule such asbis(4-β-hydroxyethoxy) diphenylsulfone,4,4'-bis(β-hydroxyethoxy)diphenyl ether, 1,2-bis(4-β-hydroxyethoxyphenyl) ethane,2,2-bis(4-β-hydroxyethoxyphenyl)propane,1,1-bis(4-β-hydroxyethoxyphenyl) cyclohexane, orp-bis(β-hydroxyethoxy)benzene.

It is important that in the copolyesters of this invention, the amountof the ester unit from hydroquinone be limited to 5-35 mole% per 100mole% of the sum of the ester units (a) and (b). If the amount is lessthan 5 mole%, the chemical resistance is not sufficiently improved.Amounts exceeding 35 mole% result in partial crystallization of thepolymer and thus adversely affect the transparency of the polymer. Thepreferred content of the ester unit from hydroquinone is 7 to 30 mole%,especially 10 to 25 mole%.

The amount of the ester unit which partly replaces the ester unitderived from terephthalic and/or isophthalic acid, and of the ester unitwhich partly replaces the ester unit from the bisphenol of formula (I)should not exceed 20 mole%. Amounts in excess of this limit will resultin various inconveniences such as reduced thermal stability, chemicalresistance, and crack resistance.

The copolyesters of this invention should have a reduced viscosity of atleast 0.5, preferably at least 0.6.

The copolyesters of this invention have markedly improved crackresistance with regard to their degrees of polymerization. The superiorcrack resistance is such that for example, when a flexural stress of 234kg/cm² is exerted on a sample polymer in carbon tetrachloride at roomtemperature, the relation between the time K (seconds) that elapsesuntil cracks occur in the sample and the reduced specific viscosity(η_(sp/c)) of the polymer satisfies the following expressions,

when η_(sp/c) is less than 1.0,

log K ≧ 2.9 (η_(sp/c)) - 0.42; and

when η_(sp/c) ≧ 1.0,

K > 300;

preferably,

when η_(sp/).c < 0.8,

log K ≧ 2.9 (η_(sp/c)) + 0.16; and

when η_(sp/c) ≧ 0.8,

K > 300.

in other words, the copolyester of this invention shows a high crackresistance of at least 21 seconds, preferably at least 80 seconds at arelatively low η_(sp/c) of, for example, 0.60.

When a polyester from an aromatic dicarboxylic acid such as terephthalicor isophthalic acid and an aromatic diol such as bisphenol is to beprepared by a melt-polymerization method, it is difficult to increaseits degree of polymerization. Since the copolyesters of this inventionexhibit superior crack resistance with relatively easily attainabledegrees of polyermization as described above, the invention has theadvantage that it is not necessary to increase the degree ofpolymerization by going so far as to employ a difficult method.

The copolyesters of this invention show improved resistance to solventssuch as acetone. The solvent resistance is such that when for example, asample is immersed in acetone at room temperature for 1 day, thepercentage (light transmission retention) of the light transmittance ofthe immersed sample based on the light transmittance of the samplebefore immersion is at least 50%, preferably at least 60%.

The copolyesters of this invention further have a heat distortiontemperature of at least 130° C, preferably at least 140° C, and thushave superior thermal stability. In addition, they have satisfactorymechanical properties such as impact strength equal to or better thanthat of polycarbonate and superior tensile strength, and also superiortransparency.

The copolyester of this invention can be prepared by reactingterephthalic and/or isophthalic acid or ester-forming derivativesthereof, hydroquinone, and the bisphenol of general formula (I) orester-forming derivatives thereof. The other dicarboxylic acid or itsester-forming derivative illustrated hereinabove can be conjointly usedin an amount of up to 20 mole% of the terephthalic acid, isophthalicacid or their ester-forming derivataives. Furthermore, the other diol orits ester-forming derivative illustrated hereinabove can be used in anamount of up to 20 mole% of the bisphenol of formula (I) or itsesterforming derivative.

Examples of the ester-forming derivatives of terephthalic and/orisophthalic acid or the other dicarboxylic acid which are used in thereaction are aryl esters and acid chlorides of these acids, the arylesters being preferred. Specific examples of the aryl esters are phenylesters, tolyl esters, xylyl esters and naphthyl esters, the phenylesters being preferred.

The terephthalic acid, isophthalic acid, or the ester-formingderivatives thereof may be used either alone or as an admixture of twoor more.

Examples of the ester-forming derivatives of hydroquinone, the bisphenolof formula (I) and the other diols which are used in the reaction areesters formed between these diols and monocarboxylic acids containing 1to 8 carbon atoms, for example, acetic acid esters.

According to the reaction of this invention, the acid component and thediol component are reacted at 200° to 500° C, preferably 230° to 320° Cat atmospheric, elevated or reduced pressures, and then the reactionmixture is melt-polymerized at reduced pressure. When the resultingpolymer has a high melting point, it is preferred to subject it furtherto solid-phase polymerization. When the solid-phase polymerization isperformed, the melt-polymerization is carried out until the reducedviscosity of the reaction product reaches 0.1-0.4, and if desired, thereaction product is heat-treated at a temperature of, say 150° to 300° Cto crystallize it, after which the solid reaction product in the form ofpowder or chips is heated at a temperature of 200° to 320° C in a streamof an inert gas or at reduced pressure to perform the solid-phasepolymerization.

One recommendable metod comprises esterifying a free dicarboxylic acidcomponent, i.e., terephthalic and/or isophthalic acid, and the otherdicarboxylic acid if used, with a compound having one hydroxyl groupdirectly bonded to an aromatic ring, i.e. an aromatic monohydroxycompound such as phenol, cresol or naphthol, and after the degree of theesterification has reached at least 80%, adding the diol component tothe reaction system. Hydroquinone, one member of the diol component, maybe present in the reaction system from the outset together with thearomatic monohydroxy compound. Where up to 20 mole%, based on thebisphenol, of an aliphatic diol is used, the aliphatic diol may bepresent in the reaction system from the outset together with thearomatic monohydroxy compound.

In one aspect of the method involving reacting the aromatic monohydroxycompound with the free acid component at the beginning, the acidcomponent is reacted with the aromatic monohydroxy compound and thehydroquinone in the first step, and in the second step, the bisphenol isreacted with the reaction product of the first step.

According to another aspect of the process, the aliphatic diol andhydroquinone can be used either in the second step or both in the firstand second steps, and according to still another aspect, the use of thealiphatic diol can be omitted.

Surprisingly enough, it has been found in accordance with the presentinvention that the polyesterforming reaction in the second stepsubsequent to the esterification reaction in the first step proceedswithout any trouble, and particularly, the presence of phenol does notadversely affect the reaction in the second step.

Hydroquinone and its ester-forming derivative are used in a total amountof 5 to 35 mole% based on the acid component irrespective of whetherthey are present in the reaction system from the outset or added to thereaction system during the reaction.

When the aliphatic diol is used in the first step, its amount is notmore than 20 mole% based on the acid component. When the aliphatic diolis used in the second step, its amount is the same as in the first step.Where it is used both in the first and second steps, its total amount inthese steps should be adjusted to within the above range.

The amount of the aromatic monohydroxy compound is such that the totalamount of it and the diol component is at least 210 mole% based on theacid component. When the amount is less than 210 mole%, the rate of thereaction is slow. The amount is preferably at least 230 mole%, morepreferably at least 250 mole%. The upper limit is about 1,000 mole%.

The total amount of the bisphenol and hydroquinone is 98 to 130 mole%,preferably 100 to 120 mole%, based on the acid component. When it isless than 98 mole% or more than 130 mole%, the rate of polymerizationbecomes slow.

In the first step of the process of this invention, the reaction iscarried out preferably at a temperature of 200° to 350° C (morepreferably 230° to 320° C) and a pressure of 1 to 15 kg/cm² (absolute).In order to perform the reaction more effectively, water generated as aresult of the reaction is distilled off from the reaction system, andthe pressure is adjusted according to variations in the partialpressures of the aliphatic diol and the aromatic monohydroxy compound orthe partial pressure of the aromatic monohydroxy compound, and the vaporpressure of water.

The reaction in the first step is carried out until at least 80 % of thecarboxyl groups of the acid component react. When the conversion of thecarboxyl groups is less than 80 %, the reaction of the reaction productwith the bisphenol to be added does not fully proceed, and polyestershaving the desired properties cannot be obtained. The preferredconversion of the carboxyl groups (the degree of esterification) is atleast 85%, more preferably at least 90%.

Following the first step, the bisphenol, or bisphenol and hydroquinone,or hydroquinone, bisphenol and aliphatic diol are added to the reactionproduct of the first step, and the mixture is polycondensed (secondstep). Preferably, the polycondensation reaction is carried out at atemperature of 200° to 350° and a pressure of 760 to 0.001 mmHg.Usually, the reaction in the second step is carried out in the moltenstate. However, the solid-phase polymerization method describedhereinabove may be used.

Preferably, a catalyst is used in the preparation of the copolyesters ofthis invention by the melt reaction described hereinabove. Examples ofthe catalyst are elemental metals such as sodium, potassium, lithium,calcium, magnesium, barium, tin, strontium, zinc, iron, aluminum,cobalt, lead, nickel, titanium, manganese, antimony or arsenic, andcompounds of these metals such as their oxides, hydrides, hydroxides,halides, inorganic acid salts, organic acid salts, complex salts, doublesalts, alcoholates, or phenolates. Of these, titanium compounds such astitanium tetrabutoxide, titanium oxalate or titanium oxide, tincompounds such as dibutyltin oxide, antimony compounds such as antimonytrioxide, and lead compounds such as lead oxide are preferred. Theamount of the catalyst is usually 0.001 to 5 mole% based on the acidcomponent. The catalyst may be added to the reaction system at an earlystage of the reaction.

While preferred embodiments of preparing the copolyesters of thisinvention by melt reaction have been described hereinabove, it should beunderstood that the copolyesters of this invention can be prepared alsoby other melt reaction procedures, for example, a method which comprisesreacting aryl esters of terephthalic and/or isophthalic acid and theother dicarboxylic acid if used with hydroquinone and bisphenol and theother diol if used, a method which comprises reacting terephthalicand/or isophthalic acid with hydroquinone, bisphenol and a diarylcarbonate, a method which comprises reacting acid chlorides ofterephthalic and/or isophthalic acid and the other dicarboxylic acid ifused, with hydroquinone and bisphenol and the other diol if used insolution, and a method which comprises reacting terephthalic acid withdiacetates of hydroquinone and bisphenol.

In the present invention, additives such as catalysts, stabilizers orcoloring agents generally used in the preparation of polyesters can beemployed without any consequent trouble.

The present invention can afford polyesters having a high degree ofpolymerization, superior crack resistance, high thermal stability, andsuperior dimensional stability, transparency and chemical resistance.

The following Examples and Comparative Examples illustrate the inventionin greater detail. In these examples, all parts are by weight. Thevarious properties given in these examples were measured by thefollowing methods.

Reduced specific viscosity (η_(sp/c))

Measured at 35° C using a mixed solvent consisting of phenol andtetrachloroethane in a weight ratio of 60:40 while maintaining theconcentration (C) at 1.2 g/deciliter.

Carboxyl number

Measured in accordance with the Conix's method (Macro. Molecular. Chem.,26, 226, 1958).

Crack resistance

An injection-molded plate-like article of the polymer, about 3.2 mmthick, about 10 mm wide and about 100 mm long, is placed on two edgesapart from each other by 80 mm. Then, a load of 2 kg is exerted on thecentral part of the molded article, and the entire molded article isimmersed in carbon tetrachloride while it is under flexural stress. Thetime that elapses until cracks occur in the molded article is measured,and expressed in seconds.

Light transmittance retention

An injection-molded plate-like article, about 3.2 mm thick, is used as asample. The light transmittance of the sample is measured by a Poicintegral spherical ultrafine turbidimeter (SEP-TU type, a product ofNippon Seimitsu Kogaku Kabushiki Kaisha). Then, the sample is immersedin acetone at room temperature for 1 day, and then its lighttransmittance is measured. The light transmittance retention (%) iscalculated from the light transmittance of the sample before immersionin acetone and that after immersion in acetone.

Amount of hydroquinone copolymerized

The amount (mole%) of hydroquinone copolymerized in the polymer isdetermined by high resolving power nuclear magnetic reasonancespectroscopy.

Heat distortion temperature

Measured in accordance with ASTM D-648.

Degree of esterification in the first step

Calculated in accordance with the following equation. ##EQU1## whereinM_(A), M_(B) and M_(C) are the molecular weights of the dicarboxylicacid component, the diol component and the aromatic monohydroxycompound, respectively (when any one of these components is a mixture oftwo or more compounds, its molecular weight is an average molecularweight of these compounds), B is the molar ratio of the diol componentto the dicarboxylic acid component at the time of feeding the startingcomponents, and CV is the carboxyl number of the reaction productmeasured after removing the excess of the aromatic monohydroxy compoundfrom it.

EXAMPLE 1

A reactor equipped with a stirrer was charged with 318.0 parts ofdiphenyl terephthalate, 22.0 parts of hydroquinone, 193.8 parts ofbisphenol A and 0.071 part of stannous acetate, and they were reacted atabout 280° C for 60 minutes. The phenol generated was distilled off fromthe reaction system. The pressure of the inside of the reaction systemwas reduced gradually, and in 15 minutes, the pressure was adjusted toabout 0.5 mmHg (absolute). At this reduced pressure, the reaction wascarried out for amother 30 minutes. During this time, the reactionproduct solidified. It was taken out, and pulverized to a size of about20 mesh on a Tyler mesh. The pulverized solid product was subjected tosolid-phase polymerization at about 0.5 mmHg (absolute) for 30 minutesat 260° C, and then for 4 hours at 280° C. The resulting polymer had areduced specific viscosity of 0.93.

The polymer was melted at about 370° C, and injection-molded through adie at about 100° C to form a sample plate having a size of about 3.2 mm× about 10 mm × about 50 mm. The resulting molded article wastransparent, and when it was allowed to stand in acetone at roomtemperature for 1 day, it remained stable without any change. The heatdistortion temperature, light transmittance, light transmittainceretention and crack resistance of the sample were measured, and theresults are shown in Table 1.

Comparative Example 1

Using 254.4 parts of diphenyl terephthalate, 63.6 parts of diphenylisophthalate, 239.4 parts of bisphenol A, and 0.071 part of stannousacetate, a polymer was prepared in the same way as in Example 1. Theresulting polymer had a reduced specific viscosity of 0.95. Aninjection-molded article of this polymer obtained in the same way as inExample 1 was transparent. However, when it was immersed in acetone, itssurface whitened in 5 minutes, and the molded article lost transparency.The properties of the sample were measured in the same way as in Example1, and the results are shown in Table 1.

Example 2 and Comparative Example 2

A flask equipped with a stirrer was charged with 159.0 parts of diphenylterephthalate, 159.0 parts of diphenyl isophthalate, 19.8 parts ofhydroquinone, 198.4 parts of bisphenol A and 0.068 part of titaniumtetrabutoxide, and they were reacted at 280° C under atmosphericpressure for 60 minutes. Then, the pressure of the inside of thereaction system was gradually reduced, and in 30 minutes, the pressurewas adjusted to about 0.5 mmHg (absolute). At this pressure, thereaction was continued for an additional 4 hours to afford a polymerhaving a reduced specific viscosity of 0.77.

For comparison, the same procedure as above was repeated except that theamount of hydroquinone was changed to 3.3 parts, and the amount ofbisphenol A, to 228.0 parts. A polymer having a reduced specificviscosity of 0.75 was obtained.

These polymers were each injection-molded into plate-like articles.After immersion in acetone at room temperature for 1 day, the sample ofExample 2 was transparent, but the sample of Comparative Example 2whitened and became non-transparent

The properties of these samples were measured in the same way as inExample 1, and the results are shown in Table 1.

Comparative Example 3

Using 254.4 parts of diphenyl terephthalate, 63.3 parts of diphenylisophthalate, 44.0 parts of hydroquinone (40 mole% based on the sum ofdiphenyl terephthalate and diphenyl isophthalate), 148.2 parts ofbisphenol A, and 0.068 part of titanium tetrabutoxide, the same reactionas in Example 2 was carried out. The reaction mixture became turbidduring the reaction, and the resulting polymer had a reduced specificviscosity of 0.51.

A molded article obtained from this polymer under the same conditions asin Example 1 was non-transparent. The properties of the sample moldedarticle were measured in the same way as in Example 1, and the resultsare shown in Table 1.

EXAMPLE 3

A polymer having a reduced specific viscosity of 0.72 was prepared inthe same way as in Example 2 except that 318.0 parts of diphenylisophthalate was used instead of 159.0 parts of diphenyl terephthalateand 159.0 parts of diphenyl isophthalate, and 116.6 parts of biphenol Zand 99.2 parts of bisphenol A were used instead of 198.4 parts ofbisphenol A. A molded article was prepared from the polymer and testedin the same way as in Example 1. The results are shown in Table 1.

                                      Table 1                                     __________________________________________________________________________    Polymer          Properties of molded article                                       Amount of             Light                                                   hydroqui-  Heat dis-                                                                           Light                                                                              trans-                                                  none co-   tortion                                                                             trans-                                                                             mittance                                                                           Crack re-                                          polymerized                                                                              temper-                                                                             mittance                                                                           retention                                                                          sistance                                     Run No.                                                                             (mole %)                                                                             η.sub.sp/c                                                                    ature (° C)                                                                  (%)  (%)  (seconds)                                    __________________________________________________________________________    Example                                                                             19     0.93                                                                              172   81   88   >300                                         Compara-                                                                      tive                                                                          Example                                                                              0     0.95                                                                              171   80   14   40                                           1                                                                             Example                                                                       2     17     0.77                                                                              165   77   83   >300                                         Compara-                                                                      tive                                                                          Example                                                                              3     0.75                                                                              164   78   16   17                                           2                                                                             Compara-                                                                      tive                                                                          Example                                                                             42     0.51                                                                              161   16   --   >300                                         3                                                                             Example                                                                       3     17     0.72                                                                              163   75   82   >300                                         __________________________________________________________________________

EXAMPLES 4 to 6

An autoclave equipped with a rectification column and a stirrer wascharged with 83.0 parts of terephthalic acid, 83.0 parts of isophthalicacid, 376 parts of phenol and 0.10 part of titanium tetrabutoxide, andpurged with nitrogen. The mixture was heated to 285° C while thepressure of nitrogen was adjusted so that the absolute pressure of thereaction system was maintained at 7.0 kg/cm². In 15 minutes, thedistillation of water began. The reaction was performed further for 200minutes while the absolute pressure was gradually reduced down to 5.5kg/cm². During this time, about 30 parts of water was distilled off. Apart of the reaction product was taken out, and after removing theexcess of phenol, its carboxyl number was measured. It was found to be735 eq/10⁶ g (the degree of esterification 89%)

The product was transferred to a reactor equipped with a stirrer, andbisphenol A and hydroquinone were added in the amounts shown in Table 2.The mixture was reacted for 60 minutes at 280° C. at atmosphericpressure. Then, the pressure of the reaction system was reducedgradually, and in 30 minutes, it was reduced to about 0.5 mmHg(absolute). At this pressure, the reaction was further carried out for180 minutes to afford polymers having the reduced specific viscositiesshown in Table 2.

Each of the polymers was dried, melted at 330° C, and injection-moldedthrough a die at 130° C to form sample plates having the same dimensionsas in Example 1. The properties of the samples were measured in the sameway as in Example 2, and the results are shown in Table 2.

                                      Table 2                                     __________________________________________________________________________                      Polymer   Properties of molded article                                        Amount                                                                        of hydro-             Light                                                   quinone          Light                                                                              trans-                                                                             Crack                                 Amount of                                                                           Amount of                                                                            copoly-   Heat dis-                                                                            trans-                                                                             mittance                                                                           resist-                               bisphenol                                                                           hydroqui-                                                                            merized   tortion                                                                              mittance                                                                           reten-                                                                             ance                             Example                                                                            A (parts)                                                                           none (parts)                                                                         (mole %)                                                                            η.sub.sp/c                                                                    temp. (° C)                                                                   (%)  tion (%)                                                                           (seconds)                        __________________________________________________________________________    4    216.6 12.1   10    0.76                                                                              164    62   82    280                             5    182.4 27.5   24    0.74                                                                              164    65   94   >300                             6    166.4 35.2   30    0.73                                                                              163    66   95   >300                             __________________________________________________________________________

EXAMPLES 7 to 10

An autoclave equipped with a rectification column and a stirrer wascharged with 83 parts of terephthalic acid, 83 parts of isophthalicacid, hydroquinone in the varying amounts shown in Table 3, 282 parts ofphenol and 0.119 part of stannous acetate, and purged with nitrogen. Themixture was heated to 285° C while the nitrogen pressure was adjusted sothat the pressure of the reaction system was maintained at 8.0 kg/cm²(absolute). Fifteen minutes later, the distillation of water began.Further, the reaction was carried out for 300 minutes while graduallyreducing the pressure down to 5.0 kg/cm². During this time, water wasdistilled off. A part of the resulting product was taken out, and afterremoving the excess of phenol, its carboxyl number was measured. Fromthe carboxyl number, the degree of esterification was calculated, andthe results are shown in Table 3.

The reaction product was then transferred to a reactor equipped with astirrer, and bisphenol A in the varying amounts shown in Table 3 wasadded. The mixture was reacted for 60 minutes at 280° C at atmosphericpressure. Then, the pressure of the reaction system was graduallyreduced, and in 30 minutes, to about 0.5 mmHg (absolute). At thispressure, the reaction was carried out for an additional 240 minutes.The reduced specific viscosities of the resulting polymers are shown inTable 3.

Each of the polymers was dried, melted at 300 to 330° C, andinjection-molded through a die at 100° C to form sample plates havingthe same dimensions as in Example 1.

The properties of the samples were measured in the same way as inExample 1, and the results are also shown in Table 3.

                                      Table 3                                     __________________________________________________________________________                           Polymer   Properties of molded articles                                           Amount           Light                                                        of hydro-        trans-                                 Amount of                                                                           Degree of       quinone                                                                             Heat dis-                                                                           Light                                                                              mittance                                                                           Crack                             hydroqui-                                                                           esteri-                                                                             Amount of copoly-                                                                             tortion                                                                             trans-                                                                             reten-                                                                             resist-                           none  fication                                                                            bisphenol merized                                                                             tempera-                                                                            mittance                                                                           tion ance                         Example                                                                            (parts)                                                                             (%)   A (parts)                                                                           η.sub.sp/c                                                                    (mole %)                                                                            ture (° C)                                                                   (%)  (%)  (seconds)                    __________________________________________________________________________    7    14.3  96    205.2 0.74                                                                              11    164   71   81    250                         8    22.0  94    184.7 0.78                                                                              18    165   66   83   >300                         9    27.5  95    182.4 0.72                                                                              23    163   68   86   >300                         10   33.0  96    166.4 0.72                                                                              29    163   70   92   >300                         __________________________________________________________________________

EXAMPLE 11 AND COMPARATIVE EXAMPLE 4

An autoclave equipped with a rectification column and a stirrer wascharged with 166 parts of terephthalic acid, 5.2 parts of neopentylglycol, 22.0 parts of hydroquinone, 376 parts of phenol and 0.1 part oftitanium tetrabutoxide, and purged with nitrogen. Then, whilemaintaining the pressure of the reaction system at 7.5 kg/cm² (absolute)by adjusting the nitrogen pressure, the mixture was heated to 280° C.After the distillation of water began, the pressure of the reactionsystem was gradually reduced down to 5.0 kg/cm² (absolute), and thereaction was performed for 300 minutes. During this time, about 33 partsof water was distilled off. A part of the resulting product was takenout, and after removing the excess of phenol, its carboxyl number wasmeasured. It was found to be 227 eq/10⁶ g (the degree of esterification96% ).

The reaction product was transferred to a reactor equipped with astirrer, and 193 parts of bisphenol A was added. The mixture was reactedfor 60 minutes at 280° C under atmospheric pressure. Then, the pressureof the reaction system was gradually reduced, and in 30 minutes, toabout 0.5 mmHg (absolute). At this pressure, the reaction was performedfor an additional 90 minutes. The resulting polymer was taken out,solidified, and pulverized.

The resulting granular polymer (about 10 mesh on a Tyler mesh sieve) wasfed into a reactor, and subjected to solid-phase polymerization for 60minutes at 260° C. and 0.5 mmHg (absolute), and then for 120 minutes at300° C.

For comparison, a polymer was prepared in the same way as in Example 11except that 228 parts of bisphenol A was used, but hydroquinone was notused.

Each of the polymers obtained was injection-molded while maintaining themelting temperature at 360°-370° C and the mold temperature at 140° C toproduce sample plates having the same dimensions as in Example 1, andthe samples were tested in the same way as in Example 1.

The reduced specific viscosities of the polymers and the test resultsare shown in Table 4.

                                      Table 4                                     __________________________________________________________________________           Polymer   Properties of molded articles                                                            Light                                                  Amount of   Heat dis-                                                                           Light                                                                              trans-                                                 hydroquinone                                                                              tortion                                                                             trans-                                                                             mittance                                                                            Crack                                            copolymerized                                                                             tempera-                                                                            mittance                                                                           retention                                                                           resistance                                  Runs (mole %)                                                                              η.sub.sp/c                                                                    ture (° C)                                                                   (%)  (%)   (seconds)                                   __________________________________________________________________________    Example                                                                            18      0.97                                                                              161   63   82    >300                                        11                                                                            Compara-                                                                      tive  0      1.01                                                                              169   67   12    38                                          Example                                                                       __________________________________________________________________________

What we claim is:
 1. A linear copolyester having ( 1) a reduced specificviscosity of at least 0.5, measured at 35° C using a mixed solventconsisting of phenol and tetrachloroethane in a weight ratio of 60:40while maintaining a concentration of 1.2 g/deciliter for thecopolyester, and (2) a light transmittance retention of at least 50%after immersion for 1 day in acetone at room temperature,saidcopolyester comprisinga. an ester unit derived from at least onearomatic dicarboxylic acid selected from the group consisting ofterephthalic acid and isophthalic acid, and hydroquinone, and b. anester unit derived from at least one aromatic dicarboxylic acid selectedfrom the group consisting of terephthalic acid and isophthalic acid, anda bisphenol of the formula ##STR2## wherein R₁ and R₂, independently ofeach other, represent alkyl containing 1 to 6 carbon atoms, and may belinked to each other,the proportion of said ester unit (a) being 5 to 35mole % based on the total of ester units (a) and (b).
 2. The copolyesterof claim 1 wherein up to 20 mole% of the bisphenol component in theester unit (b) is replaced by at least one aliphatic diol selected fromthe group consisting of ethylene glycol, neopentyl glycol andcyclohexane dimethylol.
 3. The copolyester of claim 1 wherein therelation between the crack resistance K (seconds) measured in carbontetrachloride at room temperature under a flexural stress of 234 kg/cm²and the reduced specific viscosity of the polymer (η_(sp/c)) satisfiesthe following expressions:when η_(sp/c) < 1.0, log K ≧ 2.9 (η_(sp/c)) -0.42, and when η_(sp/c) ≧ 1.0, K >
 300. 4. The copolyester of claim 1wherein said bisphenol in the ester unit (b) is2,2-bis(4-hydroxyphenyl)propane.
 5. The copolyester of claim 1 wherein atotal of up to 20 mole% of the aromatic dicarboxylic acid component inthe ester units (a) and (b) is replaced by at least one dicarboxylicacid selected from the group consisting of naphthalenedicarboxylic acid,diphenyldicarboxylic acid, methylterephthalic acid,diphenoxyethanedicarboxylic acid, methylisophthalic acid,diphenyletherdicarboxylic acid, diphenylsulfonedicarboxylic acid,succinic acid, adipic acid, sebacic acid, hexahydroterephthalic acid,hexahydroisophthalic acid, p-hydroxybenzoic acid and hydroxycaproicacid.
 6. The copolyester of claim 1 wherein the proportion of the esterunit (a) is 10 to 25 mole% based on the total of the ester units (a) and(b).